The response of volunteers to ts mutants of influenza A virus and M. pneumoniae was evaluated. The influenza A/Georgia/1974 ts-1(E) recombinant was shown to be safe, satisfactorily attenuated and antigenic in adults who lacked serum HI antibody but possessed neuraminidase immunity. This recombinant represents the 3rd influenza A virus into which the ts-1(E) lesions have been transferred. Significantly, each of the 3 ts-1(E) recombinants was suitably attenuated and antigenic. This experience supports the thesis that influenza A viruses can be predictably attenuated by transfer of a defined set of ts genetic lesions. Ts H43 mutant of M. pneumoniae was found to retain some residual virulence. An occasional seronegative volunteer developed mild afebrile bronchitis. The ts-H28 mutant of M. pneumoniae, which is more temperature sensitive than t-H43, had low infectivity for seronegative adults.